Thereto, treatment laser radiation is focused within the corneal tissue, i.e., below the tissue surface, in such a way that optical breakthroughs occur in the tissue.
Thereby, a number of processes take place in temporal sequence which are initiated by the laser radiation. If the power density of the radiation exceeds a threshold, an optical breakthrough occurs which produces a plasma bubble in the material. After the optical breakthrough occurs, said plasma bubble grows due to expanding gases. If the optical breakthrough is not maintained, the gas produced in the plasma bubble is absorbed by the surrounding material and the bubble disappears. However, this process takes very much longer than the forming of the bubble. If plasma is produced at a material boundary layer, which can also lie within a material structure, a material removal from the boundary layer occurs.
This is called photoablation. In the case of a plasma bubble, which separates previously connected material layers, it is usually called photodisruption. For reasons of simplification, all such processes shall herein be combined under the term optical breakthrough, i.e., said term not only includes the actual optical breakthrough but also the resulting effects in the material.
For great accuracy of a laser-surgical method it is absolutely essential to ensure high localization of the effect of the laser beams and to avoid collateral damages in the adjacent tissue as much as possible. Therefore, it is customary in prior art to apply pulsed laser radiation, wherein the threshold value for the power density, required for the triggering of an optical breakthrough, is exceeded only in the individual pulses. In this respect, U.S. Pat. No. 5,984,916 clearly shows that the spatial area of the optical breakthrough (in this case, the produced interaction) greatly depends on the pulse duration.
A high focusing of the laser beam in combination with very short pulses in the femtosecond range therefore allows for a precisely accurate introduction of the optical breakthrough in the material.
The use of pulsed laser radiation has recently become generally accepted particularly in ophthalmology for the laser-surgical correction of defective vision.
Defective vision of the eye is frequently caused because the diffractive properties of cornea and lens do not effect optimal focusing on the retina.
The above-mentioned U.S. Pat. No. 5,984,916 as well as U.S. Pat. No. 6,110,166 describe methods for producing incisions by means of a suitable creation of optical breakthroughs with the use of fs lasers; as a result, the refractive properties of the cornea can eventually be specifically affected. A multitude of optical breakthroughs is sequenced in such a way that a lenticular partial volume (lenticle) is isolated within the cornea of the eye. Then the lenticular partial volume, separated from the remaining corneal tissue, is removed from the cornea through a laterally opening incision. The shape of the lenticle is selected so that after removal, the form and therefore the refractive properties of the cornea are altered in such a way that the desired correction of the defective vision is effected. The cut surfaces required thereto are curved which requires a three-dimensional adjustment of the focus. Therefore, a two-dimensional deflection of the laser radiation is thereto combined with a simultaneous focus adjustment in a third spatial direction. In order to remove the lenticle, the opening incision must be relatively large, generally almost as large as the diameter of the lenticle, or even in the form of a flap, i.e., a thin slice of the cornea which is folded back in order to remove the lenticle positioned below.
In DE 10 2007 019 813 by the applicant, it was suggested that only relatively small opening incisions but, in return, a plurality of opening incisions be provided and the lenticle be removed through said incisions. However, this has proven relatively difficult because the lenticle, despite its small thickness (max. 200 μm), exhibits a certain stiffness which makes it difficult to remove the lenticle through the small opening incisions.
WO 2004/105661 describes another type of lenticle removal, wherein the lenticle is cut into small fragments by means of the fs laser (i.e., the treatment laser) in such a way that said fragments can be suctioned off by means of one or more cannulas. As a result, a smaller incision for inserting the cannula and/or cannulas suffices in comparison to the removal of the entire lenticle. However, for this purpose, the lenticle must be cut very accurately into very small pieces which significantly prolongs the duration of treatment during which the eye must be kept immobilized relative to the treatment laser in order to correctly perform the incisions.